You might check out The Torque Arm Picture Thread for many DIY ideas. There are also posts about torque plates, clamping / pinching dropouts, etc., in many threads over the years. John in CR posted pics of a set of those he made and welded to tubes that slip over the fork stanchions (after removing the original fork dropouts); I believe he then clamped and epoxied the tubes in place to the stanchions.
If you want a premade one that would probably work, check out ebikes.ca 's new torque arm varieties. One on each axle end might work, if your fork design allows it.
Keep in mind that if it's a suspension fork, "jamming" or "sticking" of the suspension action during acceleration and braking is common. If it's a lightweight fork (magnesium, carbon, etc) the materials may not be able to handle the repeated stresses of those, and could snap off without warning while riding. This can happen at the steerer/crown interface, or anywhere between that point and the axle.
